Method for production of high brightness high strength wood pulps



Feb. 6, 1962 J. SCHUBER METHOD FOR PRODUCTION OF' HIGH BRIGHTNESS HIGH STRENGTH WOOD PULPS Filed Aug. 5, 1960 Iooz man.

Now.:

United States Patent O,

3,020,197 METHOD FOR PRODUCTION OF HIGH BRIGHT- NESS HIGH STRENGTH WOOD PULPS John Schuber, Syracuse, N.Y., assignor to Allied Chemical Corporation, New York, N.Y., a corporation of New York Filed Aug. 5, 1960, Ser. No. 47,619 Claims. (Cl. 162-78) This invention relates to the bleaching of ligneous cellulosic material and more particularly to the production of bleached cellulosic material having a very high brightness, improved color stability and high strength characteristics substantially unchanged from those of the unbleached cellulosic material.

Considerable effort has been put forth with the goal of achieving a very high brightness pulp, viz. a pulp having brightness above 90 while avoiding considerable loss of strength characteristics of the pulp. Single-stage, twostage and multi-stage bleaching processes have been tried heretofore in an effort to obtain this high brightness pulp with retention of the pulp strength, but by and large the processes have not been successful. In the first place, to attain a brightness in excess of 90 has been found extremely difficult to accomplish with a brightness barrier generally being encountered between 86 and 90. Secondly, even when this high brightness pulp in excess of 90 has been produced, it has been by and large at the expense of the strength characteristics of the pulp due to oxidation and degradation of the cellulose. Apparently as the ligneous and other non-cellulosic color-forming irnpurities are oxidized and removed from the pulp as the pulp approaches higher values of brightness, the cellulose becomes more susceptible to oxidation and degradation. Thirdly, when this high brightness has been obtained in one procedure, the bleaching was disadvantageous from a commercial standpoint because requiring long time periods for the reaction typically of more than one day and as long as 20 days.

One object of the present invention is to provide a process for production of bleached cellulosic material, for instance wood pulp, having a very high brightness 4and yan improved color stability.

Another object is to provide a process for production of bleached cellulosic material, for instance wood pulp, having high strength characteristics substantially unchanged from those of the unbleached pulp.

Another object is to provide a process for the production of superbleached high strength wood pulp, for instance wood pulp, having a minimum G E. brightness of about 93 with retention of high strength characteristics substantially unchanged from those of ythe unbleached pulp.

A further object is to provide a process for the production of superbleached high strength wood pulp having strength characteristics unchanged from those of the unbleached pulp and having materially higher brightness and color stability than is possessed by bleached pulp obtained by methods currently practiced in the pulp and paper industry.

A further object is to provide a process for the production of superbleached high strength wood pulp which is yeconomical and eiiicient.

Additional objects and advantages will be apparent as the invention is hereafter described in detail.

I have carried out considerable experimental work on the bleaching o-f wood pulp in an effort to attain a high brightness pulp having a brightness in excess of 9() without substantial oxidation and degradation of the cellulose With attendant reduction in strength characteristics of the cellulose. I have tried different procedures, including 3,020,197 Patented Feb. 6, 1962 ICC single-stage, two-stage and multi-stage procedures in an effort to achieve this goal but the results have usually been the same, with the product pulp either having too low a brightness considerably below 90, its strength characteristics materially impaired or both of these undesirable results.

In accordance with the present invention, it was found that by treating the unbleached ligneous cellulosic material by a novel procedure including a particular sequence of chlorination and oxidative brightening stages together with a careful observance of pH conditions, times of treatment and pulp consistencies, a product superbleached cellulosic material was attained having a G.E. brightness in excess of and generally of about 93 or over together with high strength characteristics substantially unchanged from those of the unbleached cellulosic material. Additionally, the product bleached cellulosic material had an improved color stability.

The accompanying drawing diagrammatically illustrates the process of the invention.

The process of the invention involves, in the sequence specified, adding elemental chlorine, for instance chlorine gas to an aqueous suspension of the unbleached ligneous cellulosic material such as unbleached wood pulp containing a substantial amount of ligneous and other non-cellulosic color-forming impurities in a irst stage to chlorinate at a pH value not greater than pH 6, preferably within the range of pH 1.5-3.5 a major portion of the ligneous and color-forming impurities. The chlorinating is carried out in the first stage for a time not in excess of about 90 minutes, preferably about 30-75 minutes. An alkaline hypochlorite, for instance an alkali metal or alkaline earth metal hypochlorite, for example sodium or calcium hypochlorite, is then added to the cellulosic material in a second stage at a cellulosic material consistency of about 9-17%, preferably about l2-l7% :to oxidize a portion of the residual ligneous yand non-cellulosic colorforming impurities to considerably brighten the pulp. The pH is maintained in the second stage at a value greater than pH 8, preferably at a Value within the range of pH 940.5, and the time of treatment in the second stage is not in excess of 240 minutes, prefer-ably about 30-240 minutes. Chlorine dioxide is added to the thus-treated cellulosic material at a cellulosic material consistency of about 9-l7%, preferably about 12-l7% in a third stage to oxidize an ladditional portion of the residual ligneous and non-cellulosic color-forming impurities. The oxidizing is carried out with chlorine dioxide in the third stage for a time not in excess of 300 minutes, preferably about 60-300 minute-s and at a pH value not greater than pH 6.5, preferably Within the range of about pH 2-6.5 to brighten the pulp to a greater unit of brightness than in the second stage. The cellulosic material is then treated with an alkaline peroxide, preferably an `alkaline hydrogen peroxide solution at a cellulosic material consistency of about 9-l7%, preferably about l217% in a fourth stage to oxidize an additional portion of the residual ligneous and color-forming impurities to brighten the cellulosic material to a greater unit of brightness than in the third stage and impart color stability to the cellulosic material. The pH is maintained in the fourth stage yat a value greater than pH 8, preferably at a value within the range pH 9.0-l0.5, and the time of treatment in the fourth stage is not in excess of 300 minutes, preferably -about -300 minutes. Chlorine dioxide is then added to the cellulosic material at Ia cellulosic material consistency of about 917%, preferably about 12-17%, in a fifth stage to oxidize remaining ligneous and color-forming impurities. The pH in the fifth stage during the oxidizing is at a pH value not greater than pH 5.5, preferably within the range of pH 2,-5.5 and the time of treatment with chlorine dioxide in the fifth stage is not in excess of 300 minutes, preferably within the range of about 120-300 minutes. The product cellulosic material from the fifth stage is characterized by having a G.E. brightness in excess of 90, typically a minimum G.E. brightness of about 93, high strength characteristics substantially unchanged from those of the unbleached cellulosic material including very favorable tear, bursting and tensile strengths, and an improved color stability.

A General Electric refiectometer was used for measuring the brightnesses of the present invention. Brightness measurements were obtained of the various pulps in the form of pulp pads. Bursting strengths were measured by the Mullen test, tear strengths by the Elmendorf test, tensile strengths by the Schopper test, and fold by the M.I.T. Double Fold test, these tests being well known in the paper and pulp industry.

The process is a considerable improvement over prior art processes for bleaching ligneous cellulosic material by Virtue of (1) achieving very high brightness with a minimum G.E. brightness of typically about 93; (2) retention of high strength characteristics (in addition to the very high brightness) substantially unchanged from those of the unbleached wood pulp including very favorable tear, bursting and tensile strengths (the raising of the brightness even one unit at such high brightness levels in itself being a considerable achievement); (3) production of superbleached high strength wood pulp having materially higher brightness, whiteness and purity than bleached pulps obtrained by methods currently practiced in the pulp and paper industry; (4) production of superbleached wood pulp having color stability and retention of unbleached paper-making characteristics which are an improvement over bleached pulps currently produced in the pulp and paper industry; and (5) by being an efficient and economical process not requiring processing times of more than one day or of several days. It was surprising and unexpected that the oxidizing could be carried out in the oxidative brightening stages of the process without substantial oxidation and degradation of the cellulose.

'I'he pH values are brought or maintained within the ranges specified for the various stages to avoid certain undesirable disadvantageous results. For instance, at pH values much below pH 8 in the second stage, more than 80% of the available chlorine present tends to react as hypochlorous acid (HOCl) with less than 20% acting as the desired hypochlorite ion (OCl-). With such high concentrations of hypochlorous acid and low concentrations of hypochlorite ion, there is a marked increase in oxycellulose formation with attendant decrease in pulp strength. At pH values considerably higher than pH 5.5 in the third and fifth stages, there is considerable weakening of the cellulose due to oxidation and degradation due to depolymerization of the cellulose. Fourth stage peroxide decomposes at lower pH values with no concomitant increase in pulp brightness. At higher pH values, alkali darkening of the pulp results which nullifies effect of peroxide bleaching. The specific sequence of stages is also of utmost importance for the reason that other procedures including single, tWoand multi-stage procedures involving different sequences of stages were tried 4and found not successful.

In co-filed application Serial No. 47,611, filed August 5, 1960, entitled Production of High Brightness High Strength Wood Pulps, there is disclosed and claimed a process for bleaching ligneous cellulosic material involving generally the steps of chlorination, oxidation with an alkaline hypochlorite, oxidation with chlorine dioxide, alkali extraction and again oxidation with chlorine dioxide in the sequence stated.

In a specific embodiment of the invention, unbleached wood pulp, for instance unbleached kraft, sulfite, soda or dissolving grades of pulp containing a substantial amount of ligneous and non-cellulosic color-forming impurities is mixed with water to form a Suspension having a low pulp consistency of typically about 2-4%, preferably 3%. Chlorine gas is then passed into the aqueous pulp suspension in a first stage to chlorinate at a pH within the range of about pH 1.5-3.5 and at temperature of about 65 95 F., preferably about 70-90 F., a major portion of the ligneous and non-cellulosic color-forming impurities. The amount of chlorine added in the first stage will genr erally range from about 40%-70% of the chlorine demand of the pulp as determined by the permanganate number (T.A.P.P.I. standard method T 214 M42). Amounts of chlorine substantially higher than 70% and much below 40% of the chlorine demand should be avoided as amounts substantially higher than 70% of the chlorine demand will degrade the cellulose, while Iamounts much lower than 40% of the chlorine demand will produce a shivy hard to bleach pulp for the oxidative stages. The time of chlorination in the first stage is that which is sufficient for the exhaustion of the chlorine with a maximum time of minutes. After chlorination in the first stage, the pulp is separated from the liquor, for instance by fil-tration and then thoroughly washed with water to remove reaction products including water soluble, chlorinated ligneous and non-cellulosic color-forming impurities, unreacted chlorine and hydrochloric acid.

An alkali metal or alkaline earth metal hypochlorite, for instance sodium or calcium hypochlorite is then added to the partially purified pulp having a pulp consistency of about 12-17% from the first stage in a second stage, an oxidative brightening stage, to oxidize residual ligneous and non-cellulosic color-forming impurities. If desired, potassium or magnesium hypochlorites could be used instead of the sodium or calcium hypochlorite. The oxidizing with the alkaline hypochlorite in the second stage is carried out for a time of about 60-240 minutes at a pH value within the range of about pH 8-10.5 and at a temperature of about 90-l25 F. Excess alkali usually is added to the pulp in this second stage to insure maintenance of the high pH within the range of about 8-10.5 during the reaction. With pH values much below pH 8, for instance at pH 6 7, more than 80% of the available chlorine present tends to react as hypochlorous acid (HOCl) with less than 20% acting as the desired hypochlorite ion (OCl-). Presence of such high concentrations of hypochlorous acid and low concentrations of hypochlorite ion at the pH values substantially below pH 8 result in a marked increase in oxycellulose formation and a resultant decrease in pulp strength. Temperatures of about -l25 F. are preferably employed in the second stage with pulp consistencies of about 12-17 for purposes of increasing the reaction rate in the second stage. After the oxidative treatment in the second stage, the pulp fibers are separated from the liquor by, for instance filtration and then thoroughly washed with water.

Chlorine dioxide is then added to the partially brightened pulp at a pulp consistency of about 12-17% in a third stage, also an oxidative brightening stage, to oxidize an additional portion of the residual ligneous and colorforming impurities. Treatment with chlorine dioxide is carried out in the third stage for a time of about 60-300 minutes and at a pH value within the range of about pH 2-6.5. At pH values considerably higher than pH 6.5, considerable weakening of the cellulose occurs due to oxidation and degradation due to depolymerization of the cellulose. Temperature is about -180 F., preferably about 165 F. in the third stage. The chlorine dioxide may be added either as an aqueous solution or as a gas diluted with an inert gas, for instance nitrogen. The pulp is then separated from the liquor, for instance by filtration, and again thoroughly washed with water to remove the oxidized ligneous and non-cellulosic colorforming impurities.

The pulp is then treated with an alkaline peroxide, preferably an alkaline hydrogen peroxide solution in a fourth stage, an oxidative brightening and color stability imparting stage, at a pulp consistency of about 12-17% to selectively oxidize an additional portion of the remaining ligneous and color-forming impurities to further brighten the pulp. The treatment with the alkaline peroxide solution is carried out in said fourth stage for a time of about 60-300 minutes and at a pH within the range of about 9.010.5. The hydrogen peroxide can be employed as such in the alkaline'solution or it can be prepared from an inorganic peroxide, for example sodium peroxide, barium peroxide, potassium peroxide, or sodium perborate inasmuch as these materials generate hydrogen peroxide in `aqueous solution. The peroxide solution should contain an alkali, preferably sodium hydroxide, for purpose of pH control. However, other materials which generate alkali in aqueous solution can be used, for example sodium carbonate, trisodium phosphate and sodium pyrophosphate. Stabilizer is advantageously employed in the peroxide solution and the preferred stabilizer is sodium silicate either alone or together with magnesium sulfate also as a stabilizer.

Temperatures of about 125 -l80 F., preferably 155 l 65 F. are employed during the fourth stage peroxide oxidation. The fourth stage not only cooperates with the remaining stages of the process for providing the high brightness but also imparts improved color stability to the pulp. The pulp is then separated from the liquor by filtration and again washed with water.

The further purified and brightened pulp of improved color stability is then treated in a fifth stage, also an oxidative brightening stage, with chlorine dioxide to oxidize ligneous and non-cellulosic color-forming impurities that still remain in the pulp, typically oxidizing substantially all of the remaining impurities. Chlorine dioxide is added to the pulp at a pulp consistency of about 12-17% in this fifth stage and the treatment with the chlorine dioxide is carried out for a time of about 60-300 minutes at a pH value of about pH 2-5.5, and at a temperature of about 140-180 F., preferably about 155-165 F. pH values substantially above pH 5.5 should be avoided as they result in degradation of the ce1- lulose due to depolymerization with attendant considerable weakening of the cellulose. The pulp is separated from the liquor, for instance by filtration and again washed with water to remove oxidized ligneous and colorforming impurities. The product pulp from this fifth stage has a typical minimum G.E. brightness in the case of the kraft pulp of about 93, high strength characteristics substantially unchanged from those of the unbleached pulp including substantially unchanged very favorable tear, bursting and tensile strength and an irnproved color stability. The sequence of process steps is preferably completed within a period of not in excess of 20 hours to guard against material oxidation and degradation of the cellulose due to depolymerization with attendant considerable reduction in strength characteristics of the pulp, which tends to occur at substantially longer periods 0f treatment.

The following examples illustrate the invention. Parts and percentages are by weight unless otherwise specified.

Example I (a) Wood chips composed of about 75% white oak, 121/z% gum and 1212% poplar and containing about 37% moisture were charged to a stainless steel-lined digester. The chips were cooked with cooking liquor utilizing the kraft process for 120 minutes to 110 p.s.i.g. and 173 C., and for 115 minutes at 110 p.s.i.g. and 173 C. The cooking liquor contained 52.20 g.p.l. active alkali as Na20 and 13.05 g.p.l. NagS in the active alkali, which corresponded to a sulfidity of 25%. The active alkali concentration in the digester was 44 g.p.l. as Na20. After cooking, the cooked chips were washed in the digester, blown from the digester, and thoroughly disintegrated in a drum using a propeller-type agitator. A five minute blowdown time was employed. The resulting pulp was screened through a standard 0.014" cut flat plate screen,

centrifuged to about 30% consistency, crumbed and stored in an air-tight polyethylene bag. The undiluted black liquor from the blow had a specific gravity of 1.114 at 60 F., total solids content of 20.3%, and residual active alkali (as NazO) content of 14.6 g.p.l.

(b) Unbleached kraft pulp obtained as described in part a above had a G.E. brightness of 22.1%, a KMnO., number of 12.3, a C12 demand of 5.95%, and the following slowness and strength characteristics:

Time (minutes) Slowness Burst Tear Tensile Fold Chlorine was added to an aqueous suspension of this unbleached kraft pulp at a pulp consistency of about 3% in a first stage at a temperature of 77 F. The amount of chlorine added was 3.57% and the amount of chlorine consumed was 3.11%. The chlorination was carried out for a time of minutes and the final pH was 2.7. After separating the pulp from the liquor by filtration and washing with water the pulp had a G.E. brightness of 38.4%. Sodium hypochlorite was then added to the pulp in a second stage at a pulp consistency of 12% and a temperature of F. Sodium hydroxide was also added for the purpose of pH control in the amount of 0.5%. The amount of available C12 added as sodium hypochlorite was 1.3% and the amount of this C12 consumed was 1.0%. The second stage bleaching was continued for minutes, and the final pH was 10.8. After separation of the pulp from the liquor by filtration and washing With water, the pulp had a G.E. brightness of 72.0%.

Chlorine dioxide was then added to the pulp in a third stage at a pulp consistency of about 12% to oxidize the remaining ligneous and non-cellulosic color-forming impurities. The amount of chlorine dioxide added was 0.75% and the amount consumed was 0.69%. The treatment with chlorine dioxide was continued in ythe third stage for about 300 minutes and at a temperature of about F. The final pH was 3.3. The pulp was separated from the liquor by fil-tration and washed with Water. It had a G.E. brightness of 88.5%. The pulp was then treated with alkaline hydrogen peroxide in a fourth stage at a pulp consistency of 12%. 'Ihe amount of hydrogen peroxide added was 0.5 and the amount consumed was 0.1%. The treatment with hydrogen peroxide was carried out in the fourth stage for about 300 minutes and at temperature of about 160 F. The final pH in the fourth stage was 10.5. Sodium hydroxide was added to the mixture in the fourth stage in amount of about 0.5% for purposes of pH control, and sodium silicate and magnesium sulfate were also added to the mixture in the amounts of about 2% and 0.05% respectively for maintaining the solution alkaline. Residual ligneous and color-forming impurities were oxidized by the hydrogen peroxide in the fourth stage to further brighten the pulp. After separation of the pulp from the liquor and washing with water, ythe pulp had a G.E. brightness of 90.5%.

Chlorine dioxide was added to the pulp from the fourth stage in a fifth stage at a pulp consistency of about 12%, the amount of chlorine dioxide added being about 0.5% and the amount consumed being about 0.4%. The treatment With chlorine dioxide was carried out in the fifth stage for a time of labout 300 minutes and at a temperature of about 160 F. to oxidize remaining ligneous and non-cellulosic color-forming impurities. The final pH in the fifth stage was 4.1. The pulp was then separated from the liquor by filtration and again washed with water. A product bleached kraft pulp having a G.E. brightness of 93.1 and of improved color stability was obtained. The cupriethylene diamine viscosity of this high brightness kraft pulp was 11.6 centipoises indicating substantially un- 7 changed paper making characteristics. Following are slowness and strength characteristics for this type bleached kraft pulp having an 11.6 viscosity value:

A pine kraft pulp obtained by procedure similar to that of Example I(a) had a G.E. brightness of 23.2%, a KMnO4 number of 16.2, a C12 demand of 8.2, and the following slowness and strength characteristics:

Time (minutes) slowness Burst Tear Tensile Fold Chlorine was added to an aqueous suspension of such unbleached pulp at a pulp consistency of about 3% in a first stage at temperature of 77 F. The amount of chlorine added was 4.92% and the amount consumed was 4.69%. The chlorination was carried out for a time of 60 minutes, andthe nal pH was 2.2. The pulp was then separated from the liquor by filtration and washed with water. It had a G.E. brightness of 26.1%

Sodium hypochlorite was then added to the pulp in a second stage at a pulp consistency of about 12%. The treatment with sodium hypochlorite was carried out in the second stage for 120 minutes and at a temperature of 102 F. Sodium hydroxide was added to the mixture in the second stage in amount of about 1% for purposes of pH control. The final pH was 9.7. After separation of the pulp from the liquor by filtration and water washing of the pulp, the pulp had a G.E. brightness of 70.8%.

Chlorine dioxide was then added to the pulp in a third stage at a pulp consistency of about 12%, the amount of chlorine dioxide added being about 0.75% and the amount consumed being about 0.69%. The treatment with chlorine dioxide was carried out in the third stage for 300 minutes and at a temperature of 160 F. to oxidize remaining ligneous and color-forming impurities. The final pH was 3.1. After separation of the pulp from the liquor by filtration and washing the pulp with Water, the pulp had a G.E. brightness of 87.2%.

Hydrogen peroxide was then added to the pulp from stage three in a fourth stage at a pulp consistency of about 12%, the amount of hydrogen peroxide added being about 0.5% and the amount of hydrogen peroxide consumed being about 0.2%. Sodium hydroxide was also added in an amount of about 0.5% for purposes of pH control, and sodium silicate and magnesium sulfate were also added to the mixture in amount of about 2% and 0.05% respectively for maintaining the alkalinity of the solution. The hydrogen peroxide bleaching was carried out in stage four for a time of 300 minutes and at a temperature of about 160 F. The final pH was 10.3. The pulp was then separated from the liquor by filtration and washed with water. It had a G.E. brightness of 90.3%.

Chlorine dioxide was then added to the pulp in a fth stage at a pulp consistency of about 12%, the amount of chlorine `dioxide added being about 0.5% and the amount consumed being about 0.4%. The chlorine dioxide bleaching was carried out in stage tive for 300 minutes and at a temperature of about 160 F. The iinal pH was 5.3. After separation of the pulp from the liquor by filtration and water washing of the pulp, the product bleached kraft pulp had a G.E. brightness of 93.7% and was of improved color stability. Again the pulp viscosity value of 8.9 centipoises indicates substantially no change in paper making characteristics. Following are slowness and strength characteristics for this type high brightness kraft pulp.

Time (minutes) slowness Burst Tear Tensile Fold Example III Chlorine gas was added to an aqueous suspension of unbleached hardwood kraft pulp in a first stage at a pulp consistency of about 3%. The unbleached pulp prior to chlorination had a G.E. brightness of 29.4%, a KMnO4 number of 11.4, a C12 demand of 5.5, and the following slowness and strength characteristics:

Time (minutes) Slowness Burst Tear Tensile Fold Chlorine was added to an aqueous suspension of the unbleached pulp at a pulp consistency of about 3% in a rst stage at a temperature of 84 F. The amount of chlorine added was about 3.3% and the amount consumed was about 2.98%. The chlorination was carried out for about 41 minutes and at a temperature of about 84 F. The tinal pH was 2.4. After filtration separation of the pulp from the liquor and water washing of the pulp, it had viscosity of 17.1 centipoises.

Sodium hypochlorite was then added to the pulp in a second stage at a pulp consistency of about 12%. The amount of available chlorine added as sodium hypochlorite was 2.0% and the amount of the chlorine consumed was 1.61%. Sodium hydroxide was also added to the mixture in amount of about 0.5% for purposes of pH control. The treatment with hypochlorite was carried out for about 34 minutes at a temperature of about 120 F. The nal pH was 10.6. The pulp was then filtered from the liquor and washed with Water. It had a G.E. brightness of 74.9%.

Chlorine dioxide was then added to the pulp in a third stage at a pulp consistency of about 12%, the amount of chlorine dioxide added being about 0.65% and the amount consumed being about 0.51% The chlorine dioxide treatment was carried out for a time of about minutes and at a temperature of about F. The final pH Was 4.3. After separation of the pulp from the liquor by filtration and water washing of the pulp, it had a G.E. brightness of 86.7%.

Hydrogen peroxide was added to the pulp from stage three in a fourth stage at a pulp consistency of about 12%, the amount of peroxide added being about 0.5% and the amount consumed being about 0.1%. The hydrogen peroxide treatment was carried out in the fourth stage for about 98 minutes and at a temperature of about 135 F. The nal pH was 10.9. The pulp was then filtered from the liquor and washed with water. It had a G.E. brightness of 89.5%.

Chlorine dioxide was then added to the pulp at a pulp consistency of about 12% in a fifth stage, the amount of chlorine dioxide added being about 0.40% and the amount consumed being about 0.29%. The chlorine dioxide oxidation was elfected for about 116 minutes and at a temperature of about 160 F. The nal pH was 5.4.

9 The pulp was then separated from the liquor by filtration and washed with water. The product bleached pulp had a G.E. brightness of 92.8% and a viscosity of 11.3 cps. It also had the vfollowing slowness and strength characteristics:

In Examples I-III, slowness is given in Schopper- Riegler degrees, burst strength in percent points/pound/ ream-24 x 36-500, tear in grams force/pound/ream- 24 x 36-500, tensile strength in kg. force/15 mm./50 pound ream-24 x 36-500, and fold in M.I.T. double folds/ mm./50 pound ream-24 x 36-500.

Although certain preferred embodiments of the invention have been disclosed for purpose of illustration, it will be evident that various changes and modifications may be made therein without departing from the scope and spirit of the invention.

What is claimed is:

1. A staged process for the production of superbleached cellulosic material having a brightness in excess of 90 and of high strength characteristics which comprises, in the sequence specified, (A) treating an aqueous suspension of unbleached cellulosic material containing a substantial amount of ligneous and noncellulosic color-forming impurities in a tirst stage with elemental chlorine at not greater than pH 6 for not in excess of 90 minutes to chlorinate a major portion of said impurities, water washing the cellulosic material, (B) treating the cellulosic material at a consistency of about 9-17% with an alkaline hypochlorite in a second stage at greater than pH 8 for not in excess of 240 minutes to oxidize a portion of the residual ligneous and colorforming impurities to brighten the pulp without material degradation of .the cellulose, water washing the cellulosic material, (C) treating the cellulosic material at a consistency of about 9%-17% in a third stage with chlorine dioxide at not greater than pH 6.5 for not in excess of 300 minutes to oxidize an additional portion of said residual impurities without material degradation of the cellulose, water washing the cellulosic material, (D) then treating the cellulosic material at a consistency of about 9l7% in a fourth stage with an alkaline peroxide at greater than pH 8 for not in excess of 300 minutes to oxidize an additional portion of the residual impurities without material degradation of the cellulosic material to brighten the cellulosic material to a greater extent than in said third stage and impart color stability to the cellulosic material, water washing the cellulosic material, (E) treating the cellulosic material at a consistency of about 9%-17% in a fth stage with chlorine dioxide at not greater than pH 5.5 for not in excess of 300 minutes to oxidize remaining ligneous and color-forming impurities without material degradation of the cellulose, water washing the cellulosic material, and recovering 10 superbleached pulp having a G.E. brightness in excess of 90, improved color stability and high strength characteristics substantially unchanged from those of the unbleached cellulosic material.

2. A staged process `for the production of superbleached cellulosic material having a brightness in excess of about 93 and of high strength characteristics which comprises, in the sequence specified, (A) treating an aqueous suspension of unbleached cellulosic material containing a substantial amount of ligneous and noncellulosic color-forming impurities at a consistency of about 2-4% in a first stage with elemental chlorine at pH 1.5-3.5 for 30-90 minutes to chlorinate a major portion of said impurities, then separating the cellulosic material from the liquor and water washing, (B) treating the cellulosic material at a consistency of about l2-17% and temperature of about -l25 F. with sodium hypochlorite in a second stage at pH 8-11 for 30-240- minutes to oxidize a portion of the residual ligneous and color-forming impurities to brighten the pulp without material degradation of the cellulose, then separating the cellulosic material from the liquor and water washing, (C) treating the cellulosic material at a consistency of about 12-17 in a third stage with chlorine dioxide at pH 2-5.5 for -60-300 minutes to oxidize an additional portion of said residual impurities without material degradation of the cellulose, separating the cellulosic material from the liquor and water washing, (D) then treating the cellulosic material at a consistency of about l2- 17% in a fourth stage with an alkaline peroxide at pH 9.0-10.5 for 45-300 minutes to oxidize an additional portion of the residual impurities without material degradation of the cellulosic material to brighten the cellulosic material to a greater extent than in said third stage and impart color stability to the cellulosic material, then separating the cellulosic material from the liquor and water washing to remove substantially all caustic soda and impurities from the bodies of the cellulosic tibers, (E) treating the cellulosic material at a consistency of about l2-l7% in a fifth stage with chlorine dioxide at pH 2-5.5 for 60-300 minutes to oxidize remaining ligneous and color-forming impurities without material degradation 0f the cellulose, separating the cellulosic material from the liquor and water washing, and recovering superbleached pulp having a minimum G E. brightness of about 93, improved color stability and high strength characteristics substantially unchanged from those of the unbleached cellulosic material, the process Sequence of steps being completed within a period not in excess of 24 hours.

3. The process of claim 2 in which the cellulosic material is chemical wood pulp.

4. The process of claim 2 in which the cellulosic material is soda pulp.

5. The process of claim 2 in which the cellulosic material is kraft pulp.

References Cited in the file of this patent UNITED STATES PATENTS 2,661,261 McEwen Dec. l, 1953 2,779,656 Fennell Ian. 29, 1957 2,865,701 Schroeder Dec. 23, 1958 

1. A STAGED PROCESS FOR THE PRODUCTION OF SUPERBLEACHED CELLULOSIC MATERIAL HAVING A BRIGHTNESS IN EXCESS OF 90 AND OF HIGH STRENGTH CHARACTERISTICS WHICH COMPRISES, IN THE SEQUENCE SPECIFIED, (A) TREATING AN AQUEOUS SUSPENSION OF UNBLEACHED CELLULOSIC MATERIAL CONTAINING A SUBSTANTIAL AMOUNT OF LIGNEOUS AND NONCELLULOSIC COLOR-FORMING IMPURITIES IN A FIRST STAGE WITH ELEMENTAL CHLORINE AT NOT GREATER THAN PH 6 FOR NOT IN EXCESS OF 90 MINUTES TO CHLORINATE A MAJOR PORTION OF SAID TREATING THE CELLULOSIC MATERIAL AT A CONSISTENCY OF ABOUT 9-17% WITH AN ALKALINE HYPOCHLORITE IN A SECOND STAGE AT GREATER THAN PH 8 FOR NOT IN EXCESS OF 240 MINUTES TO OXIDIZE A PORTION OF THE RESIDUAL LIGNEOUS AND COLORFORMING IMPURITIES TO BRIGHTEN THE PULP WITHOUT MATERIAL DEGRATION OF THE CELLULOSE, WATER WASHING THE CELLULOSIC MATERIAL, (C) TREATING THE CELLULOSIC MATERIAL AT A CONSISTENCY OF ABOUT 9%-17% IN A THIRD STAGE WITH CHLORINE DIOXIDE AT NOT GREATER THAN PH 6.5 FOR NOT IN EXCESS OF 300 MINUTES TO OXIDIZE AN ADDITIONAL PORTION OF SAID RESIDUAL IMPURITIES WITHOUT MATERIAL DEGRADATION OF THE 